The present invention relates to apparatuses for manufacturing bonded substrates, and, more particularly, to apparatuses for manufacturing panel displays that have a predetermined gap between a pair of substrates, for example, liquid crystal displays (LCDs).
Panel displays such as LCDs with larger display areas are now being developed. Further, to improve the resolution, an increased pixel count per unit area is demanded in the panel displays. Accordingly, an apparatus for manufacturing panel displays with bonded substrates must be capable of accurately aligning the substrates even if each substrate is relatively large.
FIG. 1 is a plan view showing a portion of a prior art, active-matrix type liquid crystal panel display 10, as viewed from a color filter substrate.
The liquid crystal panel display 10 includes an array substrate 11 and a color filter (CF) substrate 16. The array substrate 11 has a plurality of pixel areas 12 that are formed in a matrix-like manner. Each pixel area 12 includes a switch element, or a thin film transistor (TFT) 13. The pixel areas 12 form a display area 14. A gate electrode of each TFT 13 is connected to a gate line (not shown). A drain electrode of each TFT 13 is connected to a data line (not shown). A source electrode of each TFT 13 is connected to a pixel electrode (not shown) formed in each pixel area 12. A plurality of data lines and gate lines are located at the periphery of the array substrate 11 and are connected to a terminal portion 15. The terminal portion 15 is connected to an external drive source (not shown).
The CF substrate 16 is smaller than the array substrate 11 by an area that substantially corresponds to the area of the terminal portion 15. The CF substrate 16 is spaced from the array substrate 11 at a predetermined interval. A cell gap is formed between the CF substrate 16 and the array substrate 11 and is filled with liquid crystal. The dimension of the cell gap (the cell thickness) is thus substantially equal to the interval between the CF substrate 16 and the array substrate 11. The CF substrate 16 includes a common electrode (not shown) and a black matrix (BM) 17, or a shielding film such as a color filter (red (R), green (G), or blue (B)) and a chrome film. The BM 17 is located at a position corresponding to certain pixel areas 12 in the display area 14 to form a contrast and shields each TFT 13, thus suppressing a light leak current. A BM periphery 18 shields the display area 14 from unnecessary light from the exterior. The array substrate 11 is bonded with the CF substrate 16 through a seal 19 that contains thermosetting resin.
A procedure for manufacturing a liquid crystal display mainly includes an array step, a cell step, and a module step. The array step includes formation of a wiring pattern and the switch elements (TFTs) 13 (in the case of an active-matrix type display) on each glass substrate 11, 16. The cell step includes alignment of liquid crystal, installation of spacers, and filling of liquid crystal in the cell gap between the substrates 11, 16. The module step includes installation of a driver IC and a backlight.
Conventionally, liquid crystal is filed in the cell gap in accordance with a vacuum method. In the method, the array substrate 11, which has the TFTs 13, is bonded with an opposed substrate, or the CF substrate 16, through the seal 19 located between the substrates 11, 16. After the seal 19 is hardened, the liquid crystal and the substrates 11, 16 are supplied to a depressurized treatment chamber. An inlet is formed in the seal 19, and the substrates 11, 16 are placed in the treatment chamber to immerse the inlet in the liquid crystal. The pressure in the treatment chamber is then increased to the atmospheric level. This introduces the liquid crystal into the gap between the substrates 11, 16. Subsequently, the inlet is sealed.
The liquid crystal may be filled in the cell gap in accordance with a drip method. In that method, the seal 19 is placed around the array substrate 11 in a frame-like shape. A predetermined amount of liquid crystal is then dripped on the array substrate 11. Subsequently, the array substrate 11 is bonded with the CF substrate 16 under depressurization. In this state, liquid crystal develops in the cell gap to fill the gap. As compared to the vacuum method, the drip method requires less liquid crystal and shortens the time consumed for the operation. This lowers the cost for manufacturing the panel display and improves mass-productivity.
However, an apparatus for manufacturing a panel display in accordance with the drip method has the following problems.
[1: Deformed Substrates, Display Defects, and Insufficient Substrate Attraction]
A conventional apparatus for manufacturing a bonded substrate holds each substrate 11, 16 with a vacuum chuck, an electrostatic chuck, or a mechanical chuck.
More specifically, when the vacuum chuck is used, each substrate 11, 16 is placed on an attraction side of a parallel surface plate. The vacuum chuck attracts a corresponding side of each substrate 11, 16 through vacuum, thus holding the substrate 11, 16. In this state, an appropriate amount of liquid crystal is dripped on the array substrate 11 with a dispenser. Subsequently, the CF substrate 16 is aligned with the array substrate 11, and the substrates 11, 16 are bonded together in a depressurized treatment chamber.
However, if the treatment chamber is depressurized sufficiently, the holding performance of the vacuum chuck is lowered and the vacuum chuck cannot hold each substrate 11, 16 in a stable manner. To avoid this, the treatment chamber cannot be depressurized optimally. Accordingly, sufficient pressure for bonding the substrates 11, 16 together does not act on each substrate 11, 16. As a result, the substrates 11, 16 are bonded together non-uniformly, thus causing a display defect in a resulting product.
The mechanical chuck holds the array substrate 11 and the CF substrate 16 with an engagement piece, such as a holder and a ring. In this case, the reactive force to the holding force of the mechanical chuck acts only in a limited part of each substrate 11, 16. This deforms the substrates 11, 16. Thus, the substrates 11, 16 are not located parallel with each other when bonding the substrates 11, 16 together. If the substrates 11, 16 are bonded in this state, the substrates 11, 16 are misaligned. This reduces the aperture ratio of each pixel (TFT) 13 or causes a problem such as a light leakage from a shielded portion.
In the case of the electrostatic chuck, voltage is supplied between an electrode formed on a parallel surface plate and a conductive film formed on each glass substrate 11, 16. This generates a Coulomb force between each glass substrate 11, 16 and the associated electrode to attract the glass substrate 11, 16 to the associated electrostatic chuck. The glass substrates 11, 16 are then placed in the treatment chamber as opposed to each other. The treatment chamber is then depressurized to bond the substrates 11, 16 together. However, in this case, glow discharge occurs between the opposed substrates 11, 16 during the depressurization of the treatment chamber. This damages a circuit or TFTs formed on each substrate 11, 16, thus causing a defective product. Further, if air is trapped between each substrate 11, 16 and the associated electrostatic chuck, the substrate 11, 16 may separate from the chuck during the depressurization of the treatment chamber.
[2: Damaged Liquid Crystal and Misaligned Substrates]
Conventionally, the seal 19 is formed of photoresist material that hardens in a relatively short time or thermosetting photoresist material that hardens when exposed to light and heat. However, when irradiating UV light to the seal 19 for hardening the seal 19, the liquid crystal in the vicinity of the seal 19 is also exposed to the light. This causes display non-uniformness near the boundary between the seal 19 and the liquid crystal.
Further, when the seal 19 is exposed to the liquid crystal before being hardened completely, a component of the seal 19 may elute into the liquid crystal, thus contaminating the same. To avoid this, an intense UV light may be used to irradiate to the seal 19 to rapidly harden the seal 19. However, in this case, the UV light is diffused by the substrates 11, 16, thus exposing the liquid crystal to the light.
Generally, exposure of liquid crystal to the UV light changes properties of the liquid crystal. Particularly, the substance's specific resistance is reduced. The liquid crystal thus cannot meet a requirement that an LCD with TFTs should have a relatively high voltage maintaining rate. That is, a liquid crystal cell's drive voltage in the exposed display portion is varied with respect to that in the non-exposed display portion, for example, the middle of the panel display. This causes display non-uniformness, particularly in half tone.
FIG. 2 shows a prior art panel display 10. The panel display 10 has a spacer frame 20 located along the periphery of each substrate 11, 16. The spacer frame 20 prevents the seal 19 in a non-hardened state from being exposed to liquid crystal 21. However, if an excessive amount of liquid crystal is filled in the gap between the substrates 11, 16, the liquid crystal flows from the gap through the spacer frame 20 (see FIG. 3). In this case, the seal 19, which is not yet hardened, is exposed to the liquid crystal, for example, at positions 22. Each dot in FIG. 3 corresponds to a position at which the liquid crystal 21 is dripped.
The substrates 11, 16 are bonded together under depressurization. Thus, if the substrates 11, 16 are exposed to the atmospheric pressure, the middle of each substrate 11, 16 is deformed, thus forming a space between the spacer frame 20 and the substrates 11, 16. In this case, the seal 19 is exposed to the liquid crystal 21, which is wet.
Further, even after the substrates 11, 16 are thermally hardened, a reactive force due to original waviness and warp of each substrate 11, 16 remains acting on the substrate 11, 16. Thus, if the seal 19 is formed of photoresist thermosetting material, the reactive force may be released when heating the substrates 11, 16 that are hardened. This misaligns the substrates 11, 16.
Also, there may be a change in the environment or condition of each substrate 11, 16 after the bonded substrates 11, 16 are exposed to the atmospheric pressure for hardening the seal 19. Further, when forming the cell gap, the substrates 11, 16 may be held in an unstable manner or may be distorted. In these cases, the opposed substrates 11, 16 are bonded together as misaligned, thus leading to a defect in the cell gap. Accordingly, it is complicated to manufacture the panel display 10 in a stable manner.
[3: Non-uniform Cell Thickness and its Effects on Substrates]
To distribute liquid crystal uniformly between the substrates 11, 16, the substance must be dripped at a plurality of positions of the substrate 11. However, since the amount of liquid crystal supplied to the substrate 11 is relatively small as a whole, the drip amount for each drip position must be adjusted accurately. Further, if there is an environmental change, for example, a temperature variation, the viscosity or volume of liquid crystal is altered. Also, drip performance may be varied among dispensers (drip devices). These factors vary the drip amount for each drip position, and the resulting cell thickness becomes non-uniform.
FIGS. 4A, 4B, 4C are cross-sectional views for illustrating uniform or non-uniform cell thickness of a liquid crystal panel display. In the liquid crystal panel display of FIG. 4A, an optimal amount of liquid crystal is supplied between the substrates 11, 16. The panel display thus has a desired cell thickness. More specifically, the array substrate 11 is optimally bonded with the CF substrate 16 through the seal 19. A plurality of spacer beads 23 ensures a predetermined cell thickness.
As shown in FIG. 4B, if an excessive amount of liquid crystal is supplied between the substrates 11, 16, the seal 19 is not pressed to a target dimension. This causes display non-uniformness near the periphery of the panel display.
If a further excessive amount of liquid crystal is supplied between the substrates 11, 16, not only the seal 19 is insufficiently pressed, but also the middle of the panel display is expanded, as shown in FIG. 4C. In this state, display non-uniformness is caused in the entire panel display.
[4: Undesired Exposure of Substrates to Liquid Crystal]
Each substrate 11, 16 includes an alignment mark of several micrometers. As described, the substrates 11, 16 are bonded together under depressurization after liquid crystal is dripped on the substrate 11. During the bonding, the substrates 11, 16 are aligned based on a camera image of the alignment marks such that the substrate 16 is not exposed to the liquid crystal on the substrate 11. If the exposure occurs, the liquid crystal adheres to the substrate 16, thus causing a non-uniform cell gap in the resulting panel display or exposure of the seal 19 to the liquid crystal.
Generally, the substrates 11, 16 must be bonded together with the accuracy of an order of several micrometers. Thus, a lens with a relatively long focus distance is needed to view the alignment marks of both substrates 11, 16 at the same time, if the substrates 11, 16 are spaced from each other. However, this lens has a complicated structure and is sometimes unavailable. This makes the bonding procedure difficult, and a defect may be caused in the bonded substrates 11, 16.
[5: Non-uniform Pressing of Substrates]
When bonding the substrates 11, 16 together, the opposed substrates 11, 16 are pressed to each other to obtain a predetermined cell thickness. Thus, each substrate 11, 16 must be maintained parallel with each other, and an equal pressure must be applied to the substrates 11, 16. More specifically, after dripping liquid crystal on the substrate 11, the substrates 11, 16 are pressed to each other in a vacuum treatment chamber. However, a pressing device, such as a hydraulic cylinder, is located outside the treatment chamber and is exposed to the atmospheric air. Thus, the atmospheric pressure corresponding to the introduction cross-sectional area of the pressing device acts on the pressed surface of each substrate 11, 16. Generally, the relationship between the operational amount of the pressing device and the pressure of the pressing device applied to each substrate 11, 16 is predetermined through an experiment. The pressure of the pressing device is controlled in accordance with this relationship. However, for example, aging of the pressing device may alter the pressure applied to the substrates 11, 16. This hampers reproducibility of the bonding. Further, the substrates 11, 16 may not be sufficiently pressed to each other.